Explain what a stable counterpart is.

Why is this change being made?

The section of the Support Handbook that describes "stable counterparts" does not explain what is meant by the term "stable counterpart." I've added a brief explanation as well as links to two other pages in the handbook that talk about the stable counterpart model that we use in GitLab.

Additionally, I fixed a grammatical error: changed "a" to "an" in one instance.

Does this MR meet the acceptance criteria?

Conformity

• Added description to this MR explaining the reasons for the proposed change, per say-why-not-just-what